Food products are usually stored in some kind of packaging, be it a film that is wrapped or folded around the food product or be in containers made of aluminium foil or plastics. The shelf life of packaged food is integrally related to its packaging and the product conditions. Retort technology systems use steam or superheated water to cook or sterilize food in its own package, thus extending shelf life and ensuring food safety. Obviously, such a production process requires packaging materials withstanding the production conditions. A widely used packaging in connection with retort food products is a semi-rigid or rigid container wherein the food product is filled and which container is subsequently closed with a lid sealed to the container thereby covering its opening. Containers and lids are manufactured as separate parts and delivered to manufacturer of the food product to be packed. The lids are usually delivered in stacks. These stacks are fed to the packaging machine that uses the lids to close the food package. Since lids are stored in the form of stacks comprising thousands of single lids, they have to be separated before such a lid is sealed to a food container. The fast and reliable separation of these lids poses a problem having its cause in an effect known as the glass plate effect.
This glass plate effect occurs when thin flat objects having a smooth surface are stacked and tend to stick to each other when these flat objects are removed from the stack in order to be used. This unintentional sticking of such thin flat object is also encountered in the case of lids used in food packaging and may seriously hamper the production process.
In order to achieve a fast and reliable separation of lids from a stack it is necessary to create some kind of surface structure. In the state of the art this has been done by embossing the lids. However, although the glass plate effect can be eliminated by embossing the lids, embossing itself causes some serious disadvantages. Since the embossing step of a lid is done after it has been printed on, the embossing causes cracks in the print layer. Depending on the inks used and the area size printed on these cracks are visible. Further, the sterilization or cooking step of the packed food product, a step usually done in retort packaging, worsens the problem since it may result in corrosion. This problem comes with inks used in UV-flexo printing, independently of the UV-flexo inks containing bisphenol A (BPA) or not. In the case of BPA free UV flexo inks the problem of embossing induced cracks is so severe that these inks cannot be used until now. However, it would be highly desirable to use BPA free UV flexo inks in food packaging as the potentially harmful BPA may migrate into packed food product thus increasing exposure of the consumer to this compound. There is a tendency towards restricting the use of BPA in food packaging, for instance France has recently banned its use in food packaging.
In addition, since the embossing causes spikes and dents that go right through the entire lid film, cracks are also found on the side of the lid film sealed to the container, the inner side of the film. In connection with “aggressive” food contents, e.g. sour or alkaline foods, the cracks result in corrosion, thus imposing limitations in the nature of the food contents. Avoiding an embossing step would significantly decrease susceptibility of lid films to corrosion caused by “aggressive” food contents.
So far, no other solution different from embossing has been employed to eliminate the glass plate effect.